As fundus imaging apparatuses designed to observe and capture two-dimensional front images and tomographic images of the retina of the eye to be examined, fundus camera, scanning laser ophthalmoscope (SLO), optical coherence tomography (OCT), and the like are well known and have long been in practical use.
These apparatuses are designed to obtain a retinal image by irradiating the retina as an imaging target with illumination light and by forming return light from the retina into an image on a light-receiving element or to obtain a tomographic image by interference with reference light. Light having a near-infrared wavelength is often used as this illumination light, which is not absorbed or scattered much by the light-transmitting living tissue (for example, the cornea, crystal lens, or corpus vitreum) in the eye to be examined.
The spatial resolution of an obtained image in the plane direction (lateral direction) of the retina (to be referred to as a “lateral resolution” hereinafter) is basically determined by the diameter of a beam spot scanned on the retina (or the numerical aperture of an optical system). In order to reduce the diameter of a beam spot focused on the retina, the light beam diameter of illumination light striking the eye to be examined (or the numerical aperture of the optical system) is increased.
The corner and crystal lens of the eye to be examined, which are mainly in charge of refracting light, are imperfect in terms of the uniformity of curved surface shape or refractive index. This causes high-order aberrations on the wavefront of light transmitted through these organs. For this reason, even if illumination light having a large light beam diameter is made to strike the retina, it is not possible to focus a spot on the retina at a desired diameter. Rather, the light beam diverges sometimes.
As a result, the lateral resolution of an obtained image decreases, and the S/N ratio of the image signal obtained by a confocal optical system also decreases. Conventionally, therefore, a thin beam having a size of about 1 mm, which is robust against the influence of aberrations of an optical system such as the cornea of the eye to be examined, is generally made to strike the retina to form a spot having a size of about 20 μm on the retina.
In order to solve such a problem, an adaptive optics technique is also being introduced to fundus imaging apparatuses. This technique is configured to sequentially measure the wavefront aberration of return light from a measurement target such as the eye due to variations in the characteristics of the target itself or measurement environment, and correct the aberration by using an active aberration correction device such as a deformable mirror or spatial light modulator. It was reported that making a thick beam having a size of about 7 mm strike the eye to be examined by using this technique can focus the beam into a spot diameter of about 3 μm, which is near the diffraction limit, on the retina by wavefront compensation, thus obtaining a high-resolution SLO or OCT image (see R. Zawadzki et al., “Ultrahigh-resolution optical coherence tomography with monochromatic and chromatic aberration correction” OPTICS EXPRESS/Vol. 16, No. 11/2008).
One major factor that makes it difficult to obtain stable images in a fundus imaging apparatus using adaptive optics is that the position of the pupil (iris) of the eye to be examined varies. This is caused because the head of an object to be examined moves back and forth and left and right, and the eyeball inevitably rotates in various ways in spite of the attempt to fix the line of sight by making the object observe a fixation lamp.
Using a bite bar can suppress variations in the position of the head, but will impose a burden on the object. For this reason, the use of a bite bar is not favorable sometimes. Furthermore, the ability to stably and continuously look at a fixation lamp varies among different individuals.
The first problem is that when the position of the head, that is, the position of the pupil of the eye to be examined, varies in a direction perpendicular to the optical axis of an eyepiece optical system, return light (reflected/backscattered light) from the retina also shifts on an aberration correction device placed at a position optically conjugate with the pupil. At this time, when wavefront feedback correction is performed in an open loop manner, since return light strikes the aberration correction device while shifting relative to the aberration correction value formed on the aberration correction device, the aberration correction residue increases, resulting in a deterioration in image quality, for example, a decrease in brightness or resolution.
The generation of an aberration correction value is generally expressed by the addition of function systems having orthogonality such as Zernike polynomials. In a closed-loop real-time aberration correction system as well, expressing this shift component will lead to a deterioration in the reproducibility of a curved surface unless using functions up to high-order functions. Limiting the number of orders for a reduction in calculation time makes it impossible to perform ideal aberration correction. This becomes a cause of the above deterioration in image quality.
Second, image obtaining illumination light is vignetted (limited). When, for example, a spot having a size of 3 μm is to be formed on the retina, it is necessary to make illumination light having a diameter of about 7 mm strike the pupil. In general, however, the diameter of the iris is about 8 mm even when it opens wide.
Assume that in this case, the head has moves by 1 mm to 2 mm with lapse of time. In this case, even if aberration correction is properly performed, the spot diameter on the retina increases to 4 to 5 μm to lead to a decrease in resolution. In addition, the amount of illumination light reaching the retina is lost by 10% to 20% due to vignetting. As a result, the brightness of the image also decreases.
Third, although the pupil of the eye to be examined, an aberration correction device, and a wavefront detector are arranged at optically conjugate positions with an afocal optical system, if return light falls outside the effective diameter of the optical system, even partly, the consistency of the wavefront at each position deteriorates. As a consequence, the feedback accuracy of aberration correction deteriorates. This leads to an increase in time for convergence or to divergence instead of convergence.
For the first problem, the following solution has been proposed in Japanese Patent No. 04510534. This technique provides a unit configured to observe the anterior ocular segment including the pupil to measure the position variation amount of the pupil in read time and to always match a correction effective region with the position of return light from the retina by following the position of the aberration correction device mounted on a mechanical stage in accordance with the calculated value.
However, this arrangement can solve the first problem but cannot solve the second problem. There is no mention about the third problem. In addition, since the aberration correction device generally has a volume of several cm3 to 10 cm3, the mechanical stage to be used inevitably has a large size, resulting in increases in the size and cost of the system. Furthermore, the vibrations of the stage may affect image quality.